The present invention relates to a control system for a four-wheel drive vehicle provided with an oil supply means and a hydraulic transfer clutch operated by the oil supplied by the oil supply means for establishing the four-wheel drive, and more particularly to a system for controlling the transfer clutch in accordance with driving conditions of the vehicle.
During the driving of a four-wheel drive vehicle, tire scraping occurs because of slight differences in effective wheel radii resulting from inevitable differences in tire inflation, tread wear or variation in loading. In addition, when the vehicle negotiates corners, braking phenomenon called "tight corner braking" may occur during sharp cornering. This is caused by the front wheels running through an arc of greater radius than that of the rear wheels and therefore tending to rotate faster than the rear wheels. This will result in an increase of tire wear and fuel consumption and decrease of driveability.
In order to eliminate such disadvantages, a four-wheel drive vehicle provided with an automatic transmission and a hydraulic transfer clutch is disclosed in U.S. Pat. No. 4,481,877.
In the four-wheel drive vehicle, the hydraulic transfer clutch is engaged by line pressure in the automatic transmission when engine load is heavy, since the line pressure increases at heavy load. When the vehicle turns a corner, the accelerator pedal of the vehicle is released, which means light load. Accordingly, the line pressure decreases to disengage the transfer clutch. Thus, the tight corner braking can be prevented.
However, a four-wheel drive vehicle without an automatic transmission must be provided with a hydraulic circuit to supply oil pressure to the transfer clutch in accordance with driving conditions of the vehicle. On the other hand, although a four-wheel drive vehicle having an infinitely variable belt-drive transmission has a hydraulic circuit, line pressure in the circuit increases in a low vehicle speed range such as sharp cornering. Accordingly, the line pressure can not be used for disengaging the transfer clutch during sharp cornering.